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Summary of Significant Accounting Policies - Warranty Program (Details) - USD ($)
$ in Thousands
 9 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Accounting Policies [Abstract]    
Warranty term (in years) 1 year  
Sales credit reserve term (in years) 1 year  
Sales and warranty reserves $ 52 $ 149